(a) Technical Field
The present invention relates to a lithium-sulfur battery with a polysulfide confining layer. More particularly, the present invention relates to a lithium-sulfur battery which can to prevent loss of polysulfide formed on the surface of a positive electrode during charge and discharge reactions, thus improving the durability of the battery.
(b) Background Art
In general, lithium-sulfur batteries have become a promising candidate for next-generation electric vehicles due to their excellent safety, low price of active materials, and discharge capacity of about 2,600 Wh/kg in spite of a relatively low discharge potential of about 2 V.
Referring to FIG. 1, a conventional lithium-sulfur battery is typically a secondary battery in which a sulfur-based compound with a sulfur-sulfur bond is used as a positive electrode active material. A carbon-based material, in which intercalation and deintercalation of alkali metal such as lithium or metal ions such as lithium ions take place, is used as a negative electrode active material. The lithium-sulfur battery generates and stores electrical energy through an oxidation-reduction reaction, in which, during a reduction reaction (i.e., during discharge), the S—S bonds are cleaved and thus the oxidation state of sulfur is reduced and, during an oxidation reaction (i.e., during charge), the oxidation state of sulfur increases and thus the S—S bonds are formed again. However, lithium polysulfide formed in the positive electrode during charge and discharge reactions is lost to the outside of the reaction area of the positive electrode, thus resulting in a reduction in durability.
In more detail, in a lithium-sulfur battery, the sulfur-sulfur chemical bonds are to gradually cleaved and converted into sulfur-lithium bonds during discharge, and lithium polysulfide (Li2Sx, x=8, 6, 4 or 2) formed during that period is a strengthened polar material and is readily bonded to a hydrophilic solvent. The lithium polysulfide dissolved in an electrolyte can be dispersed in the form of LiSx or anions (LiSx−, Sx2−). When the lithium polysulfide is dispersed from the sulfur positive electrode, it escapes from the electrochemical reaction area of the positive electrode, and thus the amount of sulfur which participates in the electrochemical reaction of the positive electrode is reduced, thus causing capacity loss. Moreover, in the repeated charge and discharge reactions, the lithium polysulfide reacts with the lithium metal negative electrode and thus the lithium sulfide (Li2S) is adhered to the surface of the lithium metal, which affects the reaction activity and reduces the potential of the battery.
Conventional techniques for solving the problem of “lithium polysulfide loss” in the lithium-sulfur batteries can be broadly classified into three categories. First, there is a method of delaying the outflow of the positive electrode active material by adding an adhesive capable of absorbing sulfur to a positive electrode composite material, in which the adhesive used includes active carbon fiber, transition metal chalcogenide, alumina, silica, etc. Second, there is a method of treating the surface of sulfur with a material containing hydroxide of a coating material, oxyhydroxide of a coating material, oxycarbonate of a coating element, or hydroxycarbonate of a coating material. Third, there is a method of confining lithium polysulfide in nano-scale capillary tubes by forming a carbon material into nanostructures.
However, the first conventional method of adding the additive capable of absorbing sulfur to the positive electrode may cause deterioration of electrical conduction, has a risk of side reactions caused by the additive, and is undesirable in terms of cost.
Moreover, according to the second method of treating the surface of sulfur with a certain material, the sulfur is lost during the treatment and it requires high cost values.
Lastly, according to the method of forming the conductive material into nanostructures, the formation process is complicated, it also requires high cost, and volume loss occurs due to the volume occupied by the carbon nanostructures. In addition, it is apprehended that the nanostructures lose their function in a rolling process during manufacturing of the battery.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.